1. Field of the Invention
The invention relates to shift control system and shift control method for a continuously variable transmission, and, more particularly to techniques for setting a target value used for obtaining a command value generated when the continuously variable transmission is shifted up or down.
2. Description of the Related Art
In a shift control system for a continuously variable transmission having a hydraulic actuator that is driven with hydraulic fluid supplied thereto or discharged therefrom so as to continuously change the speed ratio, it is well known to effect shifting of the continuously variable transmission by driving a shift control valve that adjusts the amount of hydraulic fluid flowing into or out of the hydraulic actuator, in a controlled manner.
One example of the above type of control system for a continuously variable transmission is disclosed in JP-A-2003-343709. In shift control of a belt-and-pulley type continuously variable transmission having primary pulley and secondary pulley each having a variable effective diameter and including a fixed sheave and a movable sheave, as disclosed in this publication, a drive command value for use in drive control of the shift control valve is calculated based on a feedback command value determined based on a difference between a target input shaft rotational speed and an actual input shaft rotational speed, and a feedforward command value determined based on the rate of change of a target value associated with the movable sheave of the primary pulley.
If the target value is set to increase stepwise in order to improve the shift response when the feedforward command value is obtained based on the rate of change of the target value, as disclosed in the above-identified publication, the feedforward command value may become excessively large, namely, may go beyond an appropriate range in which the command value can be generated without problems, depending upon the rate of change of the target value.
In view of the above possibility, it may be proposed to restrict changes of the target value so as to prevent excessively large changes thereof. However, if changes of the target value are restricted too much to be excessively small, the shift response may undesirably become insufficient or unsatisfactory.